1. Field of the Invention
The present invention relates to the unloading of reservoir fluid from gas wells, and more particularly to such reservoir fluid unloading that is achieved with little or no energy addition (such as pumping) requirements.
2. Background of the Related Art
Water is present in most wellbores that produce gas from a subsurface formation; such wellbores are also commonly known as gas wells. At the early stages of production the gas pressure in the gas-production tubing or conduit that is disposed in the wellbore is sufficiently large to lift the water that enters the gas-production conduit. At the top of the wellbore, commonly defined by a wellhead, gas and vapor and mist exit the gas-production conduit where the water content is easily separated from gas. As the production of the wellbore continues over time the gas pressure drops to the point where the water therein can no longer be lifted by the produced gas flow. This results in the accumulation of water in the bottom of wellbore, or more particularly at the bottom of the gas-production conduit, sometimes rising to a height of several thousand feet from the bottom. In such situations wellbore production stops and the only remedy is water extraction (unloading). This is conventionally achieved by means of pumping the water out of the wellbore, which is often prohibitively expensive.
In the last several decades several other methods of water unloading have been devised to avoid water pumping. The most commonly-used methods are:
a) Reducing the diameter of the gas-production conduit in the wellbore to increase the gas flow speed and hence lift mist all the way to the top of the wellbore. This method naturally reduces the gas-production rate and fails as soon as the gas pressure drops again below a critical limit.
b) Using surfactants such as detergents (e.g., soap) to reduce the water density by creation of foam, which is easier to lift by gas flow. These methods use consumable material and hence can be operationally expensive.
c) Using plunger lift, which is based on closing the top of the wellbore to let the gas pressure build up to a level which would make water lifting possible, followed by the sudden opening of the wellbore to allow the departure of the resulting high pressure gas and water mix. A solid cylinder is needed in this case, in order to push the water column up. This cylinder, called a “plunger” moves up and down the wellbore with every opening and closing of the wellbore, respectively. Because this method works intermittently it requires frequent shut-downs of the wellbore, which results in reduced overall production.
A need therefore exists for an unloading solution that is free of the above-mentioned limitations, as well as other limitations and problems existing in the present solutions.